Breast cancer is a fatal disease caused by the growth of cancerous cells within breast tissue. These cancerous cells form a lump, cyst, lesion, and the like that can grow at an alarming rate and, if left undetected, can even spread beyond the breast. Unfortunately, even with an increasing number of breast cancer cases reported each year, many women are still reluctant to go in for scheduled examinations or to receive treatment for non-cancerous lumps. A major reason for this reluctance is the physical and psychological discomfort that are experienced during examinations and treatments.
For example, screening mammography has been one of the primary diagnosis tools for breast cancer detection for over 30 years. However, many women find the compression of the breast required during a mammography procedure to be extremely uncomfortable.
Though more costly than x-ray mammography, MRI is more sensitive and can be used to detect lesions at an earlier stage than traditional mammography. Furthermore, MRI of the breast does not require compression of the breast like mammography.
In basic principle, when a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0) applied along an axis, typically designated the z axis of a Cartesian coordinate system, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) that is in a perpendicular plane to the axis, typically designated the x-y plane, and that is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the that x-y plane to produce a net transverse magnetic moment Mt. A nuclear magnetic resonance (NMR) signal is emitted by the excited spins after the excitation signal B1 is terminated. This NMR signal may be received and processed to form an image or produce a spectrum.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Radio frequency antennas, or coils, are used to produce the excitation field B1 and other RF magnetic fields in the subject being examined. Such coils are also used to receive the very weak NMR signals that are produced in the subject. Such coils may be so-called “whole body” coils that are large enough to produce a uniform magnetic field for a human subject or, they can be much smaller “local” coils that are designed for specific clinical applications such as head imaging, knee imaging, wrist imaging, breast imaging, and the like.
In the case of breast MRI, a local breast coil is typically employed. Typically, to arrange the breast in the coil, the woman is arranged in the prone position and the breast positioned in a local coil arranged beneath a patient bed on which the woman is laying. Thus, the breast is not compressed.
Two types of local coils are typically utilized in breast imaging and each coil design has an associated number of advantages and drawbacks. One coil type is often referred to as an “open” coil. These coils have coil elements that are arranged about an area where the breast is arranged but are disposed at a distance from the actual breast. These open coils provide ready access to the breast when it is arranged in the coil to facilitate stereotactic procedures, allow the placement of fiducial markers, and the like. Unfortunately, the distance between the exterior of the breast and the location of the coil reduces the signal-to-noise ratio (SNR). That is, the further the coil is decoupled from the breast, the lower the SNR
To improve SNR, some traditional local breast coils utilize a “saddle” design that is built into a frame that extends below the patient bed. In this regard, the coil is arranged directly about the breast. While this configuration increases SNR, these traditional saddle coils present a number of drawbacks. Specifically, the amount of access to the breast when arranged in the coil is significantly restricted. That is, saddle coils can only be rotated about the principal magnetic field (z) axis without seriously compromising SNR. For biopsy, rotation about z axis is done to improve access and visualization of the target region, but this rotation is highly constrained and limited to only a few degrees of rotation.
Therefore, it would be desirable to have a system and method for imaging a breast using an MRI system that permits ready access to any desired portion of the breast when the breast is positioned in the local coil while still providing a high SNR and not requiring an onerous registration or orientation process.